1. Field of the Invention
The present invention relates to a solid-state imaging device that converts a signal charge photoelectrically converted by a photoelectric conversion region into an electric signal and the resulting electric signal is then transferred by a transfer transistor, a method of manufacturing such an solid-state imaging device, and an imaging apparatus provided with the solid-state imaging device.
2. Description of the Related Art
FIG. 1 illustrates a schematic cross sectional diagram of a transfer transistor used in the solid-state imaging device according to the related art. As shown in FIG. 1, the transfer transistor includes a gate electrode 102 formed on a first conductive type, such as a p-type, semiconductor substrate 100 through an insulating film 101. In addition, a sidewall 103 is formed on the gate electrode 102.
In a photoelectric conversion region 108, for example, an n-type second conductor-type semiconductor region 104 is formed as a photodiode embedded in the semiconductor substrate 100. Furthermore, for example, a surface-shield region with a p-type first conductive-type semiconductor region 105 is formed on the second conductive-type semiconductor region 104. The first conductive-type semiconductor region 105 of the photoelectric conversion region 108 is provided for preventing the generation of dark current due to an influence of the interface of the semiconductor substrate 100. A second conductive-type semiconductor region 107 having an impurity concentration higher than that of a second conductive-type semiconductor region 104 is formed on the semiconductor substrate 100 in a readout region 109 to be used as a charge readout region. In addition, a second conductive-type semiconductor region 106 having an impurity concentration lower than that of the second conductive-type semiconductor region 104 is formed below the sidewall to obtain an LDD structure.
Such a transfer transistor accumulates photoelectrically converted electrons in the second conductive-type semiconductor region 104 of the photoelectric conversion region 108. When a high voltage is applied to the gate electrode 102, electrons accumulated in the second conductive-type semiconductor region 104 are transferred from the photoelectric conversion region 108 to the readout region 109.
In the transfer transistor constructed as described above, the photoelectrically converted charges are accumulated in a portion comparatively deep (i.e., a deep portion) in the second conductive-type semiconductor region 104 of the photoelectric conversion region 108. Therefore, a high voltage may need to be applied to the gate electrode 102 to complete the transfer of charges. However, it is difficult to provide the gate electrode with a high voltage when pixels are further miniaturized.
Furthermore, the first conductive-type semiconductor region 105 may become a barrier to the transfer of electrons from the photoelectric conversion region 108 to the readout region 109 in the transfer transistor constructed as described above. Thus, the first conductive-type semiconductor region 105 is not formed below the sidewall 103 except in the case of thermal diffusion in the manufacturing process. For this reason, the interface of the semiconductor substrate 100 exists in the second conductive-type semiconductor region 104 below the sidewall 103. Accordingly, dark current occurs in the second conductive-type semiconductor region 104 due to the interface of this semiconductor substrate 100, causing a defective pixel with a white spot. When the first conductive-type semiconductor region 105 is formed below the sidewall 103, it may be a transfer barrier and a still higher voltage may need to be applied to the gate electrode.
Japanese Unexamined Patent Application Publication No. 2006-49921 (JP 2006-49921 A), for example, has proposed a technology for reducing a transfer voltage in the configuration of the transfer transistor described above using an epitaxial growth method that forms a surface-shield region and an elevated source drain (ESD) region on a semiconductor substrate to reduce the transfer voltage.